FIG. 1 illustrates a conventional system 10 that that includes a transmitter 12 and a transmit observation receiver (TOR) 14 coupled to an output of the transmitter 12 via a coupler 16. As illustrated, the transmitter 12 includes a digital predistorter (DPD) 18, a Digital-to-Analog Converter (DAC) 20, an upconversion subsystem 22, and a power amplifier (PA) 24 connected as shown. An output of the transmitter 12 (i.e., an output of the PA 24) is coupled to an antenna 26 via a transmit (TX) filter 28. The output of the PA 24 is also coupled to an input of the TOR 14 via the coupler 16. The TOR 14 includes a downconversion subsystem 30 and an Analog-to-Digital Converter (ADC) 32 connected as shown. An output of the TOR 14 is coupled to a first input of a subtractor 34. The input of the transmitter 12 is coupled to an input of a delay and gain adjustment component 36, and an output of the delay and gain adjustment component 36 is coupled to a second input of the subtractor 34.
In operation, a baseband processor 38 sends a digital transmit signal to the input of the transmitter 12. The digital transmit signal is predistorted by the DPD 18 to compensate for nonlinearity of the PA 24, converted from digital to analog by the DAC 20, upconverted to a desired radio frequency by the upconversion subsystem 22, and amplified by the PA 24 to provide a radio frequency transmit signal at the output of the transmitter 12. The radio frequency transmit signal is filtered by the transmit filter 28, and the resulting filtered radio frequency transmit signal is transmitted via the antenna 26. The TOR 14 samples the radio frequency transmit signal output by the transmitter 12 to provide a digital TOR output signal at the output of the TOR 14. The subtractor 34 then subtracts a gain and delay adjusted version of the digital transmit signal input to the transmitter 12 from the digital TOR output signal to provide an error signal that represents a residual InterModulation Distortion (IMD) in the radio frequency transmit signal output by the transmitter 12. The error signal can then be utilized by the baseband processor 38 to, for example, adaptively configure the DPD 18 to compensate for the nonlinearity of the PA 24.
The delay and gain adjustment component 36 can accurately model a path between the input of the transmitter 12 and the output of the TOR 14 for conventional transmit signals. However, for modern and future wireless communications standards, the transmit signals have or will have significantly wider bandwidths. For wideband transmit signals, the delay and gain adjustment component 36 is not capable of accurately modeling the path between the input of the transmitter 12 and the output of the TOR 14. In particular, the delay and gain adjustment component 36 is not capable of accurately modeling a path between the output of the transmitter 12 and the output of the TOR 14 for wideband transmit signals. More specifically, a frequency response of the TOR 14 varies significantly over the bandwidth of the wideband transmit signals. As a result, if only the delay and gain adjustment component 36 is used to model the path between the input of the transmitter 12 and the output of the TOR 14 for wideband transmit signals, the performance of the transmitter 12 will suffer (e.g., the baseband processor 38 will take longer to adapt the DPD 18). As such, there is a need for a model of the frequency response of the TOR 14 for wideband transmit signals.